The present invention relates to the field of thin film silicon photovoltaic devices and more particularly to the use of a microcrystalline silicon film as a current collecting film in such devices.
The art of thin film solar cells is well known and improvements are being made therein in an effort to make such devices commercially practical. For example, amorphous silicon p-i-n structures are disclosed in U.S. Pat. Nos. 4,385,200 and 4,388,482 issued to Hamakawa on May 24, 1983 and June 14, 1983, respectively. Another Hamakawa et al patent, U.S. Pat. No. 4,410,559 issued Oct. 18, 1983 provides a more detailed teaching concerning the glow discharge techniques for depositing amorphous silicon films with various doping materials. Each of these patents is hereby incorporated by reference by the teachings of cell structures and methods of manufacture.
While the basic amorphous silicon p-i-n photovoltaic device seems quite simple, continued research and development efforts have identified numerous problems which degrade the performance thereof. For example, U.S. Pat. No. 4,492,736 issued to Tanner on Jan. 8, 1985, discusses the problem of contact between a current collecting film and a p-i-n thin film silicon photovoltaic device. This patent is incorporated by reference for all purposes. In particular, this patent notes that it is difficult to make a good low resistance contact between amorphous silicon and the typical back-face contact which is made of aluminum. The Tanner patent teaches the use of an n-type microcrystalline silicon film in place of the n-type amorphous silicon region normally found in thin film silicon p-i-n photovoltaic devices. The microcrystalline form of silicon forms a much better contact with the aluminum current collecting film and thereby improves device performance.
Another problem commonly encountered in the manufacture of thin film silicon devices is the occurrence of "pinholes." Pinholes are small spots or holes in the silicon film which leave the front current collecting film exposed. Therefore, when the back-face current collecting film is deposited, it makes a direct contact with the front face film and forms a shunt or short circuit. Various techniques have been developed for eliminating or fixing such shunts. The typical device structure uses a tin oxide front face contact and an aluminum back-face contact. This structure can be somewhat self fixing because oxygen from the tin oxide tends to form an insulating layer of aluminum oxide at the contact point. This reaction generally forms sufficient insulation to limit the resulting shunt current to an acceptable level in high power devices, that is devices intended for use in direct sun light. However, this type of shunt is unacceptable in devices for low light level, for example indoor, applications such as "solar" powered calculators. For such low light level use, less than 200 lux, essentially no shunting can be tolerated.
Therefore, it is seen that it would be desirable to provide a photovoltaic device structure which eliminates or is not sensitive to pinholes and a method of manufacture which eliminates or is not affected by pinholes which occur in the thin silicon film.